1. Field of the Invention
The invention relates to a method and apparatus for cleaning baghouses used in plants used, for example, for the production of hot mix asphalt, more particularly, relates to an improved method and apparatus for removing particulate materials that have caked onto the outer surfaces of the bags of such a baghouse.
2. Discussion of the Related Art
Baghouses are well known for collecting fines, soot and other particulates (commonly known collectively as dust) from exhaust streams from dryers, mixers, and other apparatus used, for example, in the production of hot mix asphalt (HMA), in soil remediation processes, in milling processes, woodworking processes, etc. The typical baghouse includes a metal enclosure in which are disposed a lower filter chamber and an upper clarified gas plenum separated by a horizontal partition extending the length of the enclosure. A plurality of bags made of cloth or another gas permeable material are suspended from the partition and have upper outlets opening into the plenum. A contaminated gas inlet opens into the filter chamber, and a clarified gas outlet is formed in the plenum and is connected to the inlet of a fan or blower drawing gases through the baghouse.
In use, dust in the gases is collected on the outer surface of the bags as the gases flow therethrough, thus clarifying the gases. The dust collected on the outer surface of the bag quickly forms a cake which at first enhances the dust removal capability of the bags but inhibits further flow of gases into the bags as the thickness of the cake increases. Some of the dust must be periodically removed to reduce the thickness of the cake to maintain the effectiveness of the baghouse. Commonly used dust removal techniques include mechanical shaking, pulsed air injection or jet pulse injection, and reverse air flow. Mechanical shaking and pulsed air injection tend to be the most effective because they highly agitate the bag structure, but both accelerate bag wear and both risk bag tearing and both require rather complex structures. Examples of employing mechanical and pulsed air or jet pulse bag cleaning assemblies are disclosed in U.S. Pat. Nos. 2,829,735 to Kroll and 3,739,557 to Anderson et al., respectively.
The effectiveness the pulse jet bag cleaning technique is limited further by the fact that contaminated gases continue to flow through the cleaned bags during or at least immediately after the bag cleaning process. Thus, much of the dust removed from a bag during cleaning is recaptured on the same or an adjacent bag a short distance from the release point. This recapture causes the bag to be relatively clean at the top while having a relatively thick layer of dust caked onto the bottom. The uneven accumulation along the length of the bag is exacerbated by the fact that larger, heavier particles fall faster than relatively fine, light particles and are more apt to cake onto the lower portion of the bags. This uneven dust accumulation results in an uneven gas velocity through the bag and a resulting variation in filter efficiency from bottom to top.
Reverse air flow, on the other hand, relies on a gentle "puffing" of the bag rather than a vigorous shaking and thus presents little risk to the bag. Indeed, some theorize that this puffing provides little if any direct agitation to the bag and that it merely serves to temporarily permit the bag, which is normally drawn up against the frame by gases flowing therethrough, to relax and form a more natural shape, thereby breaking up layers of caked particulates. The typical reverse air flow system isolates a module of many rows of bags during the cleaning process, cleans all isolated bags simultaneously, and then places all of the isolated bags back on-line when the next module is cleaned. An example of a reverse air flow bag cleaning system is disclosed in U.S. Pat. No. 3,951,627 to Barr, Jr., et al.
The effectiveness of many known dust cleaning techniques is further hindered by the fact that dust tends to accumulate differently on bags at different locations in the baghouse. For instance, particulate accumulate on the upper portions of the bags tends to be greater on the outer bags in a row and on the bags near the front of the baghouse where the gases contact the bag at greater velocities. The ratio of fine dust particles to coarse particles also tends to vary from bag to bag. Bag cleaning should preferably be optimized to accommodate variations in accumulation described to avoid overcleaning or undercleaning. Undercleaning, i.e., removing too little dust from bags, decreases the overall efficiency of the baghouse by unduly inhibiting the flow of gases through the bags. Overcleaning, i.e., removing too much dust from bags, leads to the flow of an undesirable percentage of fine particles through the bags and out of the baghouse. The need therefore exists to control the amount of cleaning along the length of each bag and from bag to bag to optimize bag cleaning. No known prior art bag cleaning systems satisfy this need.